a. Field of the Invention
The invention relates to non-contacting measurement of surfaces, and in particular to a scanning acoustic measurement system for testing flatness of wafer surfaces and the like.
b. Prior Art
Previously, non-contacting measurements of flatness for wafers and similar articles have been made optically and capacitively. With respect to optical devices, reflective and interferometer devices are known. One reflective type employs a beam which is directed onto a test surface from a position perpendicular to the surface. Small deviations in flatness cause the beam to be reflected at an angle which is not perpendicular to the surface. By measuring the angle of reflection plus the duration and extent of the reflective deflection as the beam is moved along a straight line, the deviation from flatness may be computed.
In the interferometer instrument, fringes are created by means of splitting a beam of monochromatic light from a single source into two components of which one follows a path which includes a reflection on a test surface and the other follows a path which includes a reflection on a reference surface. The path length difference causes an optical fringe pattern to appear. Any deviation in flatness causes the number of fringes to change. By counting fringes, the deviation from flatness may be measured. A problem with optical methods is that specularly reflective surfaces are required.
Another apparatus for measuring flatness of a test surface is a capacitive gauge which consists of three spaced apart reference sensors, together with an array of similar sensors. The three reference sensors are used to capacitively determine the distance to a test surface. The plane defined by those three distances is a reference plane. The array of other sensors then measures the distance to the test surface and any deviations from the reference plane indicate deviations in flatness. A problem with capacitive measurement devices is that the test surface must be at least partially conductive.
In prior U.S. Pat. No. 4,175,441, K. Urbanek, G. Kren and W. Wheeler disclose an acoustic gauge for measuring distances to planar members and the thickness of flat sheets and the like.
While prior art devices make generally accurate measurements of flatness, in most instances the time for obtaining an accurate multi-point flatness appraisal of a test object is limited. This is because most devices measure either a single point at a time or a modest number of points simultaneously, except for the laser reflective apparatus. In the case of that instrument, the laser beam scans one line across the surface, but time must be taken to determine the extent of a slope, as indicated by a variance in the angle of reflection of the beam. In the case of the interference apparatus, an entire surface may be measured at a time, but interpreting the data by counting fringes and measuring spacing between fringes is tedious.
An object of the invention is to devise a non-contacting flatness gauge wherein a flatness profile based on a large number of sample points may be obtained in a relatively brief time interval.